1. Field of the Invention
This invention relates to a thermal mass flow meter, and more particularly the invention relates to an improvement of a thermal mass flow meter having a sensor pipe defining a passage of gas flow to be measured and an electric heater heating the sensor pipe, so that the mass flow rate of the gas in the sensor pipe is determined by measuring a temperature difference between an upstream point and a downstream point of the sensor pipe.
2. Description of the Prior Art
A typical thermal mass flow meter of the prior art is schematically shown in FIG. 1. The gas flow 1 to be measured in a main pipe 2 consists of laminar flow elements 3. The pressure difference across the upstream and the downstream of the laminar flow elements 3 is proportional to the volume flow rate Q of the gas flow 1 to be measured. This pressure difference causes a minor gas flow .DELTA.Q of laminar type in a sensor pipe 4 which is a branch to the main pipe 2.
Heaters 5 and 6 are wound on the sensor pipe 4. The heaters 5 and 6 are energized by a power source 7, so that the sensor pipe 4 is heated to a certain temperature, for instance in a range of about 50.degree.-100.degree. C. The heaters 5 and 6 also act as temperature sensors, and a bridge circuit is formed by the heaters 5, 6 and resistors R.sub.1 and R.sub.2, as shown in the figure. An output voltage .DELTA.E is produced across the joint between the two heaters 5, 6 and the joint between the resistors R.sub.1, R.sub.2. When the minor flow in the sensor pipe 4 is zero, i.e., .DELTA.Q=0, the bridge circuit is balanced and its output voltage is also zero, i.e., .DELTA.E=0. When the minor flow .DELTA.Q assumes a finite value, the temperature of the upstream heater 5 is reduced while the temperature of the downstream heater 6 is increased. Accordingly, the output voltage .DELTA.E also assumes a finite value, which value is proportional to the flow rate of the gas flowing through the sensor pipe 4. Since the branching ratio .DELTA.Q/Q is constant, the mass flow rate in the main pipe 2 can be determined from the above output voltage .DELTA.E.
The reason why the minor flow .DELTA.Q in the sensor pipe 4 causes a temperature reduction in the heater 5 and a temperature rise in the heater 6 is in that the minor flow .DELTA.Q of the gas in the sensor pipe 4 transmits heat to the downstream. Such transmission of heat may occur outside the sensor pipe 4. In FIG. 1, if a weak wind blows from the heater 5 to the heater 6, heat is transmitted from the left to the right in the outside of the sensor pipe 4, and the heater 5 is cooled while the heater 6 is heated to raise its temperature, as if a minor flow .DELTA.Q was occurring in the sensor pipe 4 so as to produce an erroneous output voltage .DELTA.E. To avoid such erroneous output voltage due to wind, the heater-carrying portion of the sensor pipe is placed in a sensor housing 8.
Although the sensor housing 8 eliminates the adverse effects of the wind, it cannot solve the problem of posture error. More particularly, the initial adjustment of the thermal mass flow meter is effected while keeping the heaters 5 and 6 horizontally, and if the heater 6 is for instance located at a level higher than the heater 5 during the usage, heat is convected from the heater 5 to the heater 6, so that a drift in the zero point is caused in the flow meter, as a posture error. The magnitude of the posture error is maximized when the heater 6 is positioned immediately above the heater 5 or when the posture of the flow meter is turned by 90.degree. relative the horizontal posture. It is undesirable to have a large zero point drift for a small change of the posture. Especially, in the case of a flow meter mounted on a car, such zero point drift due to the posture error is fatal.
As a means to remove the posture error, the Japanese Patent Laying-open Publication No. 10413/82 teaches the use of an evacuated sensor housing 8 which encloses the heaters 5 and 6 as shown in FIG. 1. With the evacuated sensor housing 8, the heat leakage from the heaters 5 and 6 is mostly in the form of radiation to the outside of the sensor housing 8 and not in the form of convection, so that theoretically speaking, no posture error can occur. However, such sensor housing 8 must be made airtight even after being evacuated, so that its structure tends to be costly. Besides, an evacuating step is necessary during the manufacture. The heat conductivity of air at 1 atmospheric pressure is kept substantially unchanged even after evacuation unless the pressure is reduced below 0.01 mmHg, so that the heat conductivity is unstable unless the vacuum pressure is kept below 0.001 mmHg. It is very difficult to maintain such a high degree of vacuum for an extended period of time.
As another means for reducing the posture error due to heat convection, Japanese Patent Publication No. 23094/81 discloses the use of foamed thermal insulator 9, such as foamed polyurethane which covers the heaters 5 and 6 as shown in FIG. 1. The heat from the heaters 5 and 6 leaks to the outside of the thermal insulator 9 by way of thermal conduction and thermal radiation therethrough. Since thermal convection has nothing to do with the heat transfer in the thermal insulator 9, the posture error can be eliminated. Japanese Patent Laying-open Publication No. 110920/82 discloses a foamed thermal insulator 9 which also acts as a sensor casing 8.
The method of using the foamed thermal insulator has a shortcoming in that it has a large time delay in the change of the output voltage .DELTA.E in response to a step-like change in the minor flow .DELTA.Q. The reason for the delay in the response is in that the heat conductivity of the foamed thermal insulator 9 is smaller than that of air. In short, the thermal capacity of the foamed thermal insulator 9 is large, so that the delay in the response is caused. A step-like increase in the minor flow .DELTA.Q will reduce the temperature of the heater 5 while increasing the temperature of the heater 6, and it takes time before thermal equilibrium is reached at new temperature distribution. It takes a long time for the temperature change at the heaters to reach the outer surface of the foamed thermal insulator 9. In general, the foamed thermal insulator 9 made of organic substance has a low heat resistivity, so that the temperature of the heaters 5 and 6 cannot be raised too high. This is one of the causes for limiting the sensitivity of the flow sensor or flow meter (output voltage .DELTA.E/minor flow .DELTA.Q).